FIG. 1 shows a schematic structure of a conventional scanning tunneling microscope which is disclosed, for example, in HYOMEN KAGAKU (Surface Science), Vol. 8, No. 1 (1987), Pages 2-13. The scanning tunneling microscope shown in FIG. 1 comprises a vacuum chamber 1 a stage 3 on which a specimen 4 is disposed a probe 5 having its tip end disposed to face the specimen 4, a piezoelectric element 6, and a scanning system 8. The reference numeral 2 represents air in the chamber 1 before it is evacuated, and the reference numeral 7 represents tunneling current.
The operation of this microscope is as follows. The specimen 4 is positioned on the stage 3 which is provided with vibration isolation. The probe 5 of, for example, tungsten is disposed at a position spaced by about 10 angstroms from the surface of the specimen 4. Air 2 in the vacuum chamber 1 is exhausted in the direction indicated by an arrow A by means of an evacuation system (not shown) so that ultrahigh vacuum in the order of 10.sup.-8 torr can be achieved in the chamber 1. A negative voltage is applied to the probe 5 with the specimen 4 grounded, which causes the tunneling current 7 to flow through the gap between the probe 5 and the specimen 4. The tunneling current 7 is detected, and the probe 5 is caused to move in the direction perpendicular to the surface of the specimen 4 by means of the piezoelectric element 6 in such a manner that the tunneling current 7 is maintained constant. The length of the piezoelectric element 6 increases in proportion to a voltage applied to it (e.g. 1 mm per volt), and, accordingly, the probe 5 moves in the vertical direction in an amount proportional to that voltage. Thus, the amount of the vertical movement of the probe 5 can be determined from the voltage applied to the piezoelectric element 6. At the same time, the probe 5 is caused to move in parallel with the surface of the specimen 4 by means of the scanning system 8 to scan the surface of the specimen 4. From the thus measured amounts of movement of the probe 5 in the directions perpendicular to and parallel to the surface of the specimen 4, the surface atomic structure is determined.
In the conventional scanning tunneling microscope arranged as stated above, because the spacing of about 10 angstroms between the probe 5 and the specimen 4 is too small, the probe 5 tends to contact the specimen 4, which may cause the tip end of the probe 5 to be damaged because it is very fine. In the conventional scanning tunneling microscope, it is necessary to replace a probe each time it is damaged. In addition, each time the probe 5 is to be replaced with a new one, the ultrahigh vacuum of about 10.sup.-8 torr in the vacuum chamber 1 must be once released, and after the replacement of the probe 5, the chamber 1 must be re-evacuated to the required ultrahigh vacuum of about 10.sup.-8 torr. It is a time consuming work.